Pure

As you progress through the world tour you will unlock more 'slots', this allows you to create more ATVs. Once you have 5 slots available build a new ATV in each one, using a seperate engine, there are 5 available, these are:

Yoshimura 450cc 4 Stroke
DASA Racing 449cc 4 Stroke
FMF 450cc 4 Stroke
Sparks Racing 250cc 2 Stroke
Alba Racing 350cc 2 Stroke

Use each one and the cheevo is yours!

Combine this with the Grease Monkey achievement...
Make sure you have 5 slots free in your garage. Do a quick race build on 5 Sprint bikes. Be sure that you select with 'A' the specific engine you want on each of the 5 bikes, as 'auto-build race' will always pick the same one. I made all bikes Class D engines. This will give you the 'Mechanic' achievement.
Then race through the same sprint and win, with each of the 5 bikes you have built. I chose one of the Stage 1 sprints ('Alto Vista') as it's a very simple figure of 8 that is easily won.

Note that the Grease Monkey achievement doesn't seem to pop immediately after you win the 5th race. I went into the single race menu as mentioned on another site and the achievement popped immediately. Odd.